There is a long felt need for an economically viable process to form VAM directly from acetic acid. VAM is an important monomer in the production of polyvinyl acetate and polyvinyl alcohol products among other important uses. VAM is currently produced from two key raw materials, ethylene and acetic acid. Ethylene is predominantly produced from petroleum based raw materials although acetic acid can be produced to a lesser extent from petroleum based raw materials. Therefore, fluctuating natural gas and crude oil prices contribute to fluctuations in the cost of conventionally produced petroleum or natural gas-sourced VAM, making the need for alternative sources of VAM all the greater when oil prices rise.
It has now been found that VAM can be produced essentially from a mixture of carbon monoxide and hydrogen (commonly known as synthesis gas) involving a few industrially viable steps. For example, it is well know that synthesis gas can be reduced to methanol, which is in fact the industrially preferred way to manufacture methanol. Methanol thus formed can then be converted selectively to acetic acid under catalytic carbonylation conditions which is again the industrially preferred process for the manufacture of acetic acid. The acetic acid thus formed then can be selectively converted to ethyl acetate under suitable catalytic conditions. Although there are no known preferred processes for such a conversion, the prior art does provide certain processes for such a conversion of acetic acid to ethyl acetate albeit at low conversions and yields thus making it industrially unsuitable.
For instance, one such process involves first hydrogenation of carboxylic acids over heterogeneous catalysts to produce alcohols, which can then be converted to the corresponding acetates by an esterification reaction. For example, U.S. Pat. No. 2,607,807 to Ford discloses that ethanol can be formed from acetic acid over a ruthenium catalyst at extremely high pressures of 700-950 bar in order to achieve yields of around 88%, whereas low yields of only about 40% are obtained at pressures of about 200 bar. However such extreme reaction conditions are unacceptable and uneconomical for a commercial operation.
More recently, even though it may not still be commercially viable it has been reported that ethanol can be produced from hydrogenating acetic acid using a cobalt catalyst at superatmospheric pressures such as about 40 to 120 bar. See, for example, U.S. Pat. No. 4,517,391 to Shuster et al.
On the other hand, U.S. Pat. No. 5,149,680 to Kitson et al. describes a process for the catalytic hydrogenation of carboxylic acids and their anhydrides to alcohols and/or esters utilizing a platinum group metal alloy catalyst. The catalyst is comprised of an alloy of at least one noble metal of Group VIII of the Periodic Table and at least one metal capable of alloying with the Group VIII noble metal, admixed with a component comprising at least one of the metals rhenium, tungsten or molybdenum. Although it has been claimed therein that improved selectivity to a mixture of alcohol and its ester with the unreacted carboxylic acid is achieved over the prior art references it was still reported that 3 to 9 percent of alkanes, such as methane and ethane are formed as by-products during the hydrogenation of acetic acid to ethanol under their optimal catalyst conditions.
A slightly modified process for the preparation of ethyl acetate by hydrogenating acetic acid has been reported in EP 0 372 847. In this process, a carboxylic acid ester, such as for example, ethyl acetate is produced at a selectivity of greater than 50% while producing the corresponding alcohol at a selectivity less than 10% from a carboxylic acid or anhydride thereof by reacting the acid or anhydride with hydrogen at elevated temperature in the presence of a catalyst composition comprising as a first component at least one of Group VIII noble metal and a second component comprising at least one of molybdenum, tungsten and rhenium and a third component comprising an oxide of a Group IVb element. However, even the optimal conditions reported therein result in significant amounts of by-products including methane, ethane, acetaldehyde and acetone in addition to ethanol. In addition, the conversion of acetic acid is generally low and is in the range of about 5 to 40% except for a few cases in which the conversion reached as high as 80%.
Similarly, it has been reported in the literature that ethyl acetate can be converted to ethylene under a variety of conditions. Although some of the processes reported in the art may not be suitable for a commercial operation, certain modifications thereof maybe suitable for selective conversion of ethyl acetate to ethylene such that it can be employed industrially as further described herein in the detailed description of the instant invention.
For example, it has been reported that ethylene can be produced from various ethyl esters in the gas phase in the temperature range of 150-300° C. over zeolite catalysts. The types of ethyl esters that can be employed include ethyl esters of formic acid, acetic acid and propionic acid. See, for example, U.S. Pat. No. 4,620,050 to Cognion et al., where selectivity is reported to be acceptable.
U.S. Pat. No. 4,270,015 to Knifton describes obtaining ethylene involving a two-step process in which a mixture of carbon monoxide and hydrogen is reacted with a carboxylic acid containing 2 to 4 carbon atoms to form the corresponding ethyl ester of said carboxylic acid which is subsequently pyrolyzed in a quartz reactor at elevated temperatures in the range of about 200° to 600° C. to obtain ethylene. The ethylene thus produced contains other hydrocarbons, particularly, ethane as an impurity. It was also reported therein that the concentration of ethane can reach high values, near 5% by pyrolyzing pure ethyl propionate at 460° C. More importantly, the conversion of the esters and yield of ethylene are reported to be very low.
U.S. Pat. No. 4,399,305 to Schreck describes obtaining high purity ethylene from ethyl acetate employing a cracking catalyst composed of a perfluorosulfonic acid resin commercially sold under the trademark NAFION® by E.I. DuPont de Nemours & Co.
On the other hand, Malinowski et al., Bull. Soc. Chim. Belg. (1985), 94(2), 93-5, disclose the reaction of acetic acid on low-valent titanium heterogenized on support materials such as silica (SiO2) or titania (TiO2) resulted in a mixture of products including diethyl ether, ethylene and methane where selectivity is poor.
WO 2003/040037 discloses that crystalline microporous metalloalumino-phosphates (ELAPO), particularly, SAPO-type zeolites, such as SAPO-5, SAPO-11, SAPO-20, SAPO-18 and SAPO-34, having Si/Al ratio of 0.03-017 are useful as adsorbent or as a catalyst for the production of olefins from an oxygenated feedstock containing methanol, ethanol, n-propanol, isopropanol, C4-C20 alcohols, methyl ethyl ether, di-methyl ether, di-ethyl ether, di-isopropyl ether, formaldehyde, dimethyl carbonate, dimethyl ketone and/or acetic acid. A similar disclosure utilizes a silicoaluminophosphate molecular sieves comprising at least one intergrown phase of molecular sieve. It is reported that in this process a feedstock containing an oxygenate contacts a catalyst comprising the molecular sieve in a reaction zone of a reactor at conditions effective to produce light olefins, particularly ethylene and propylene. See U.S. Pat. No. 6,812,372 to Janssen et al. It is mentioned that such oxygenated feedstocks include acetic acid, but the disclosure appears to be limited to either methanol or dimethyl ether. See, also, U.S. Pat. No. 6,509,290 to Vaughn et al., which further discloses conversion of oxygenated feedstocks to olefins.
Bimetallic ruthenium-tin/silica catalysts have been prepared by reaction of tetrabutyl tin with ruthenium dioxide supported on silica. It has been reported that these catalysts exhibit different selectivities based on their content of tin/ruthenium ratio (Sn/Ru). Specifically, it has been reported that the selectivity for the hydrogenolysis of ethyl acetate is quite different, which depends upon the Sn/Ru ratio in the catalyst. For instance, with ruthenium alone on SiO2, the reaction is not selective: methane, ethane, carbon monoxide, carbon dioxide as well as ethanol and acetic acid are produced. Whereas, with low tin content, it has been reported that the catalysts are fairly selective for the formation of acetic acid, while at higher Sn/Ru ratios, ethanol is the only detected product. See Loessard et al., Studies in Surface Science and Catalysis (1989), Volume Date 1988, 48 (Struct. React. Surf.), 591-600.
The catalytic reduction of acetic acid has also been studied. For instance, Hindermann et al., J. Chem. Res. Synopses (1980), (11), 373, have disclosed the catalytic reduction of acetic acid on iron and on alkali-promoted iron. In their study they found that the reduction of acetic acid on alkali-promoted iron, followed at least two different routes depending on the temperature. For example, they found that at 350° C., the Piria reaction was predominant and gave acetone and carbon dioxide, as well as the decomposition products methane and carbon dioxide, whereas the decomposition products were reduced at lower temperatures. On the other hand, at 300° C. a normal reduction reaction was observed resulting in the formation of acetaldehyde and ethanol.
In addition, it should also be noted that there are industrially viable processes for the production of VAM from ethylene and acetic acid. For example, U.S. Pat. No. 6,696,596 to Herzog et al., which is incorporated herein by reference in its entirety, discloses that VAM can be produced in the gas phase from ethylene, acetic acid and oxygen or oxygen containing gases over a catalyst comprising palladium and/or its compounds, gold and/or its compounds and alkali metal compounds on a support, wherein the catalyst further comprises vanadium and/or its compounds.
There are also reports in the literature of integrated processes for the manufacture of VAM involving oxidation of an alkane such as ethane or an alkene such as ethylene to acetic acid and in a subsequent step reaction of so formed acetic acid with additional amounts of ethylene in the presence of oxygen to form VAM. See for instance, U.S. Pat. No. 6,040,474 to Jobson et al., which describes the manufacture of acetic acid and/or vinyl acetate using two reaction zones wherein the first reaction zone comprises ethylene and/or ethane for oxidation to acetic acid with the second reaction zone comprising acetic acid and ethylene with the product streams being subsequently separated thereby producing vinyl acetate. See also, U.S. Pat. No. 6,476,261 to Ellis et al. which describes an oxidation process for the production of alkenes and carboxylic acids such as ethylene and acetic acid which are reacted to form vinyl acetate demonstrating that more than one reaction zone can be used to form the vinyl acetate.
U.S. Pat. No. 6,852,877 to Zeyss et al., describes another integrated process wherein ethane is reacted with molecular oxygen in the presence of a suitable oxidation catalyst in first reaction zone; concurrently, in a second reaction zone another batch of ethane is oxidatively dehydrogenated to ethylene in the presence of a suitable catalyst and subsequently in a third reaction zone, the acetic acid formed in reaction zone 1 is reacted with ethylene produced from a second reaction zone in the presence of additional amounts of molecular oxygen and a suitable catalyst to form VAM.
From the foregoing it is apparent that existing processes do not have the requisite selectivity to form ethyl acetate from acetic acid and/or its direct conversion to ethylene and then to convert the resulting products to VAM in an integrated process thus making them industrially adoptable to produce VAM essentially from synthesis gas and/or synthesis gas based products.